1. Field of the Invention
The present invention relates to an apparatus for recording on sheets with recording means, and particularly relates to the configuration of a particular part of an ink jet recording apparatus which records by discharging ink droplets from ink jet recording means, the part being the printing region and the areas upstream and downstream therefrom.
2. Description of the Related Art
In order to record with ink jet recording apparatuses in a stable manner, the effects of wrinkling and undulations (hereafter referred to as “cockling”) upon ink being applied to recording sheets serving as a recording medium must be eliminated as much as possible, and also recording sheets which tend to curl due to the effects of humidity and the like must be kept from floating toward the head, so as to maintain the distance between the surface of the recording sheet and the recording head. This is because in the event that the recording sheet floats up so far as to come into contact with the recording head, this not only leads to soiling of the recording sheet due to ink droplets on the recording head being transferred thereto and smearing of the unfixed ink on the recording sheet, but also causes trouble such as clogging of the ink discharge orifices of the recording head.
Techniques for solving the above-described problems have been conventionally disclosed, such as in Japanese Patent Laid-Open No. 2000-071532 and Japanese Patent Laid-Open No. 2000-158644. A first conventional example according to Japanese Patent Laid-Open No. 2000-071532 will first be described, with reference to FIGS. 12 and 13.
An ink jet recording apparatus 100 comprises a transporting roller 102 serving as sheet transporting means, disposed upstream in the recording sheet transportation direction of a recording head 101, a pinch roller 103 which is pressed against the transporting roller 102 so as to be driven thereby, and sheet discharge rollers 104 and sheet discharge spurs 105 pressed against the sheet discharge rollers 104 so as to be driven thereby, which are provided downstream in the sheet transportation direction. The ink jet recording apparatus 100 further comprises a platen 106 for guiding the rear face of a recording sheet S at a position facing the recording head 101, with multiple ribs 107 and grooves 108 alternately formed on the upper face for forming waves in the sheet width direction of the recording sheet S. A sheet pressing plate 110 having protrusions 109 for guiding recording sheets S into the grooves 108 is provided upstream from the platen 106 in the sheet transportation direction. Also, wave holding spurs 112 for pressing the recording sheet S into the grooves 108 in the same way are provided downstream in the sheet transportation direction from the grooves 108. The sheet discharge rollers 104 and the sheet discharge spurs 105 are positioned downstream from each rib 107 of the platen 106 in the sheet transportation direction, and the height of the nip formed between the sheet discharge rollers 104 and the sheet discharge spurs 105 is arranged to be approximately the same height as the ribs 107.
With such a configuration, the behavior of the recording sheet S upon the platen 106 will be described with reference to FIG. 13, which is a view of the arrangement shown in FIG. 12 from the direction of the arrow C.
The recording sheet S which has passed beyond the sheet pressing plate 110 is provided with a wave shape such as indicated by the solid line S5. At this time, in the event that the recorded image is one of high concentration, recording ink which uses water as the primary medium thereof will have been ejected onto and absorbed into the recording sheet S in great amounts, so the recording sheet S becomes swelled. Now, the recording sheet S has the apexes of the waves immediately upstream from the recording area pressed between the ribs 107 and a horizontal portion 111 of the sheet pressing plate 110, and accordingly does not readily move even due to swelling. On the other hand, movement is not restricted at the multiple grooves 108, so stretching of the recording sheet S due to swelling primarily occurs at the grooves 108. The recording sheet S is displaced downwards beforehand by the protrusions 109, so cockling is sure to occur downwards, and the recording sheet S following recording assumes a form indicated by the dotted line S6. The recording sheet S upon which cockling such as indicated by S6 at the recording area is transported by the sheet discharge rollers 104 and sheet discharge spurs 105 with the cockling state maintained by the wave holding spurs 112. The sheet discharge roller pairs are disposed downstream from each rib 107, thereby being situated at the crest portion of each cockle, and accordingly maintaining the cockling state.
Also, with regard to curling of the recording sheet S due to the environmental conditions, the recording sheet S is provided with the wave shape upstream of the recording area, and the wave shape is maintained by the wave holding spurs 112 even after the trailing edge of the recording sheet S passes over the sheet pressing plate 110. Accordingly, the rigidity of the recording sheet S is increased and curling is corrected, so the recording sheet S is prevented from floating up towards to the recording head 101 side.
Next, a second conventional example according to Japanese Patent Laid-Open No. 2000-158644 will be described with reference to FIGS. 14 through 16. Description which would be repetitive of the description of the first conventional example will be omitted here.
An ink jet recording apparatus 200 comprises a transporting roller 202 serving as sheet transporting means, disposed upstream in the recording sheet transportation direction of a recording head 201, a pinch roller 203 which is pressed against the transporting roller 202 so as to be driven thereby, and sheet discharge rollers 204 and sheet discharge spurs 205 pressed against the sheet discharge rollers 204 so as to be driven thereby, which are provided downstream in the sheet transportation direction. The ink jet recording apparatus 200 further comprises a platen 206 for guiding the rear face of a recording sheet S at a position facing the recording head 201, with multiple ribs 207 and auxiliary ribs 208 shorter than the ribs 207 formed on the upper face for forming waves on the sheet width direction of the recording sheet S. The transporting roller 202 and the pinch roller 203 are arranged such that the pinch roller 203 is offset as to the transporting roller 202 in the direction of the platen 206 as shown in FIG. 15, which is a cross-sectional view along line XV—XV in FIG. 14, and with the nip portion thereof positioned above the ribs 207. Accordingly, the recording sheet S is pressed against the ribs 207 from an upper diagonal direction while being transported, and as indicated by the solid line S7 in FIG. 16, which is a view of that shown in FIG. 14 from the direction of the arrow D, the recording sheet S is provided with a wave formed wherein crests are formed at the ribs 207 and troughs are formed at the portions between the ribs 207. Also, the sheet discharge rollers 204 and the sheet discharge spurs 205 are positioned downstream from each rib 207 of the platen 206 in the sheet transportation direction, and wave holding spurs 209 for pressing the recording sheet S in between the ribs 207 and between the sheet discharge rollers 204 in the same way are also provided downstream.
With such a configuration, the behavior of the recording sheet S upon the platen 206 is such that it assumes a wave shape indicated by the solid line S7 before recording with the recording head 201 as shown in FIG. 16, and in the event that a high-concentration image has been recorded thereupon, stretching due to swelling of the recording sheet S between the ribs 207 and auxiliary ribs 208 occurs in the direction away from the recording head 201 as shown by dotted line S8.
However, there are problems with the above-described conventional examples. As a first problem, the shape of the waves and the depth of the troughs of the recording sheet change before and after the leading edge of the recording sheet reaches the wave holding spurs. The reason that this occurs is as follows. The shape of the waves, and consequently the depth of the troughs, formed upstream of the recording head by the sheet pressing plate 110 or by the offset of the pinch roller 203 as to the transporting roller 202, differ depending on the thickness and rigidity of the recording sheets, and further depending on the density of the fibers from one sheet to another, or even from one place to another within the same sheet. However, the height of the wave holding spurs 112 or 209 is set so as to be lower than the ribs 107 or 207 taking into consideration the tolerance of parts and assembly, in order to press the recording sheets between the ribs 107 or 207 in a sure manner. Accordingly, in many cases, the depth of the trough increases after the leading edge of the recording sheet reaches the wave holding spurs. Also, the wave holding spurs 209 are rotatably borne by torsion coil springs 210 so as to be capable of elastic movement, and are capable of moving a certain amount in the height-wise direction, but this arrangement is originally intended for providing wave shapes to the recording sheets, and accordingly cannot completely eliminate the effects.
Consequently, the distance between the recording head and the recording face of the recording sheet changes before and after the leading edge of the recording sheet reaches the wave holding spurs, and with multi-pass recording wherein scanning of the recording head is repeated for multiple sheet feeds for forming an image, there have been problems such as the ink droplets landing off-target, offset in image colors, overall coarseness, and so forth.
As a second problem, in the event that a high-concentration recording image is recorded on a recording sheet, swelling of the recording sheet occurs in the direction away from the recording head at the trough portion of the wave shapes of to the recording sheet beforehand, i.e., between the grooves 108 or between the ribs 207 and auxiliary ribs 208 of the platen, thereby preventing contact with the recording head, as already described with regard to the conventional art. However, this has negative effects as well, in that at portions where cockling occurs, the recording face of the recording sheet is distanced form the recording head, so the ink droplets land further off-target. Also, the massive wave effects of the region where cockling occurs act to distance the trough portions from the recording head for portions that have not been recorded on yet as well, so the ink droplets land far off-target in the same way.
Further, as a third problem, the depth of the wave shapes of the recording sheet changes between before and after the trailing edge of the recording sheet passing over the transporting roller 102 or 202. This occurs due to the entire width-wise area of the unrecorded face of the recording sheet changing from a state of being supported by the transporting roller to a state of being supported only by the ribs 107 or 207 of the platen. That is to say, in the state before the trailing edge of the recording sheet passes over the transporting roller, the entire width of the recording sheet is supported by the transporting roller at the time of providing wave shapes for troughs between the ribs 107 or 207 of the platen, which acts as drag. However, after the trailing edge of the recording sheet passes over the transporting roller, this drag does not act on the recording sheet any more, so the depth of the troughs increases. Consequently, there have been problems such as the ink droplets landing off-target, offset in image colors, overall coarseness, and so forth.